Server license confusion: help please

Hello, I am a Linux and desktop guy with absolutely zero understanding of Windows Server licensing no matter what I am trying to read here.

A business I am helping has need for a server with seemingly simple requirements. There won't be any active directory or weird stuff -- just computers in a workgroup accessing a drive with some users remote desktoping in.

Site 1 will host the server. A software app and its data be on the server. There will be six computers at site 1, each with a drive mapped from the server. Each computer will have a local instance of the software that accesses the shared mapped drive data. None of these computers will need user access to the server. This all seems simple.

Site 2 will have 4 computers. Each computer will remote desktop to the site 1 server to run an instance of the software app.

What "flavor" of server software is needed? I'm guessing 5 CALs is what I need, but I still need remote desktop licenses too...?

Windows Remote Desktop Services(RDS) licensing — Windows Remote Desktop Services (formerly known as Windows Terminal Services) is built into Windows Server 2003 and 2008, but you will still need to get a separate Windows RDS User CAL for each client that will access Terminal Services in your organization. The RDS CAL replaces the older Terminal Services (TS) CAL.

I just don't know what to get. Two kinds of licenses? One kind that I install in terminal services? Which version of server? Any input would be appreciated.

That only works if one person at a time uses the app remotely. More than one requires some sort of server / terminal server.
Assuming you use Windows terminal server you need a CAL for each concurrent user, so 4 licenses for 4 people using the app at the same time. This will also provide remote access for your users - create an account for each of them on the server. You could go to AD at this point and have each PC connect to the server, making user password management easier for you users. Use a VPN between the sites and the PCs can be domain members.

Paul pretty much covered it. But MS licensing is very complex. You'll also technically need a Server CAL for the users to access resources on the server. That part is an honor system thing. The workstations will connect to resources regardless of whether they have Serer CALs or not, these are just on paper. But if the license police ever knock on the door... Are you in the US? Give Software One, or any reputable MS licensing partner a call and run that scenario by them. They'll tell you exactly what you need.